EL CAJON – Millions of sterile male Mediterranean fruit flies were dropped by air Friday over an 11-square-mile area of El Cajon, and several residential areas were treated by ground after four of the destructive pests were found in area traps, authorities said.

The sterile Medflies were released from fixed-wing aircraft at the rate of about 250,000 per minute, said Steve Lyle, spokesman for the California Food and Agriculture Department in Sacramento.

Blanketing an affected area with sterile male flies gradually wipes out the population because female flies can't reproduce and die soon after mating, ending their life cycle.

Bob Atkins, San Diego County agricultural commissioner, said Friday that three Medflies were found in separate traps in El Cajon in an area on the southeast side of Interstate 8 near Greenfield Drive. A fourth was found about 1.5 miles away in an area near Granite Hills Drive.

State crews conducted ground treatments Friday in the three areas near Greenfield Drive, including near John F. Kennedy Park, and will do so Saturday in the area near Granite Hills Drive, Lyle said. The park itself was not affected, Lyle said.

Residents who are affected by ground treatments are notified at least 24 hours in advance, Lyle said.

“If you haven't been notified, you aren't affected,” he said.

Atkins said that a backpack sprayer is being used for the ground treatment in the residential areas. An organic compound is sprayed on individual plants, especially fruit trees that would be susceptible to Medfly infestations. The compound, called Naturalyte, is a fly bait mixed with the organic pesticide spinosad.

Atkins said Medflies are attracted to the bait, which is a naturally produced toxin registered for organic use. However, other insects such as bees are not attracted to it, he said.

“It (Naturalyte) was selected specifically to be the least toxic and most effective as possible,” Atkins said.

Asked whether people would notice the release of Medflies in the area, Atkins said: “They might see them, but you would have to recognize what they are. They have very colorful wings, and eyes with a bluish sheen.”

The Medflies are about quarter-of-an-inch long.

Medflies can attack more than 250 types of fruits and vegetables.

In San Diego County, Atkins said crops that would be at risk from either quarantine or loss include persimmons, avocadoes, apples, tomatoes and grapes, with a total value of local crops at more than $280 million.

Lyle said Friday that a quarantine in the affected El Cajon area is expected soon. Residents would be prohibited from removing fruits and vegetables from the quarantine area.